U.S. Secretary of State Hillary Clinton acknowledged on Monday that the United States and its allies are actively considering imposing a no-fly zone over Libya as a means to prevent Muammar al-Qaddafi’s government from cracking down on rebel forces as the country seemingly spirals into civil war. There have been widespread reports of Libyan Air Force jets bombing and firing on protesters; rebels reportedly shot down a plane while it was firing on an anti-Qaddafi radio station on Feb. 28. Some 200 Arab groups from throughout the Middle East signed a letter over the weekend in support of a U.N. sponsored no-fly zone. How exactly do these zones work?

It depends on the circumstances. There are two primary types of no-fly zones imposed by air forces. The first is imposed by one military over another, while the two sides are at war. In practice, this type of no-fly zone amounts to a warning from one side that it will engage the other’s aircraft if they are spotted in a given territory.

The second type, more applicable to the situation in Libya, is when an outside power possessing overwhelming air superiority restricts flights over a given country in order to discourage an internal conflict or humanitarian crisis. This is a relatively recent tactic, which was used most famously in Bosnia and Iraq during the 1990s. No-fly zones are often a compromise in situations where the international community is demanding a response to ongoing violence, but full military intervention would be politically untenable.

The establishment of no-fly zones is authorized under Chapter 42 of the U.N. Charter, which states that if non-military methods are insufficient for responding to a threat to international peace, "demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations" may be employed.

That’s pretty vague, so the actual terms and rules of engagement are set up in the resolution that authorizes any specific no-fly zone. In the case of Bosnia, "Operation Deny Flight" as it was called, was imposed by U.N. Security Council Resolution 816 in 1993, and applied to all "fixed-wing and rotary-wing aircraft in the airspace of the Republic of Bosnia and Herzegovina." The already established U.N. Protection Force was charged with monitoring the airspace as well as authorizing exceptions to the ban, such as humanitarian aid flights. Deny Flight followed an earlier, less-stringent operation — Sky Monitor — under which only military flights were banned and U.N. Forces only could only document violations, rather than engaging the aircraft.

The ban made sense for the Yugoslav war, as virtually all the fixed-wing military aircraft in the region were under the control of one side — the Bosnian Serbs. The zone was tested on Feb. 28, 1994, when six Serbian fighter planes were shot down by U.S. Air Force F-16s, in what became known as the Banja Luka incident.

The effectiveness of Deny Flight, however, is debatable. NATO credits it with removing air power as a weapon for the Bosnian Serb forces and pushing the conflict toward an earlier conclusion. Critics contend that it did little to prevent the worst abuses of the conflict, including the 1995 Srebrenica massacre. The mission was later expanded into an active NATO bombing campaign.

The other most notable examples of no-fly zones were those imposed by the United States and its allies over northern and southern Iraq following the 1991 Gulf War. Operations Northern Watch and Southern Watch (later, Southern Focus) aimed to prevent Saddam Hussein’s air force from attacking Iraq’s Kurdish and Shiite minorities. (Iraqi Mig and Mirage fighter planes were used in the 1988 gas attack on the Kurdish city of Halabja, which killed up to 5,000 people.)

Unlike Deny Flight, the Iraqi operations were never specifically authorized by the U.N. Security Council. The United States, Britain, and France claimed authority under Security Council Resolution 678, which stated that member states could use "all necessary means" to ensure that Iraq complied with its post-war disarmament obligations. However, many observers felt there was no basis in international law for the zones and the debate over their legality continues to this day.

The mechanism for enforcing a no-fly zone depends on the situation and the country doing the enforcement. In the case of Iraq, the zones were monitored by AWACS surveillance planes that would contact allied fighter jets, which flew regular missions, if a violation was detected. The operation was relatively effective — very few violations of the zones were recorded between the end of the Gulf War and the U.S. invasion of Iraq in 2003 when it was finally lifted — though human rights abuses against the southern Shiites by Iraqi ground forces continued. For those who argue that a no-fly zone would hasten Qaddafi’s departure from power, it’s also worth considering that Saddam Hussein ruled under one for over a decade. But the Iraqi experience also demonstrated the dangers of no-fly zones: In a 1994 incident, two U.S. Blackhawk helicopters were shot down by American F-15 fighters after being mistaken for Iraqi aircraft.

In the case of Libya, nearby Italy has suggested that it might allow its military bases to be used to stage the enforcement of a no-fly zone. The U.S. has its own airbase in the country as well. The United States is also positioning an aircraft carrier off the coast of Libya, which a Pentagon spokesman saidwill "provide flexibility" for future military options. The main obstacle to imposing a no-fly zone on Libya may be political, rather than military: U.N. diplomats say that 15-member Security Council is unlikely to agree to a zone unless there’s a dramatic escalation of violence by the Libyan Air Force, and this time, it’s unlikely the United States or its allies have the appetite to go it alone.

In any case, while a no-fly zone could presumably prevent Qaddafi’s planes from firing on protesters or rebel forces, it would do nothing to stop his ground forces and mercenaries from continuing their assault. Given the limited utility then, the U.S. and its allies must now decide if all the trouble involved in setting up a zone — including inevitable questions of legality — are worth the risk.

Thanks to Michael N. Schmitt, professor of public international law at Durham University Law School and former legal advisor to Operation Northern Watch.